Saturday, June 17, 2017

Onward to the main subject of this post. The seabird Petralca from the Miocene of Austria was first described in 1987 as an auk. However, this identification was (reportedly*) not justified by any anatomical observations and other researchers have subsequently suggested that Petralca may instead belong to a different group of diving birds, the loons (or divers, if you're British). Saying anything conclusive regarding Petralca has been difficult though, given that the holotype is not particularly well preserved. Some of the bones associated with the specimen are not even preserved directly, only evident as impressions.

*I cannot confirm this for myself, as the original description is in German.

The holotype of Petralca, from Göhlich and Mayr (in press).

To uncover more information about the specimen, paleontologists Ursula B. Göhlich and Gerald Mayr initiated further preparation of the fossil as well as casting of the preserved bone impressions. Armed with the new data they collected from these ventures, they were able to better compare the skeleton of Petralca to those of definite fossil loons as well as extant loons and auks.

There's no use beating around the bush: they found that Petralca is a loon. Every available skeletal element in Petralca that could be compared to those of loons and auks was more similar to those of loons. Of particular note is that the radiale (one of the wrist bones) of Petralca has a deep and prominent notch, which is a very distinctive feature of extant loons, but is absent in auks.

In addition to clearing up its phylogenetic affinities, this reassessment of Petralca also provides clues to how it lived. Extant loons can swim quickly underwater by propelling themselves with their feet, whereas previously known early Miocene loons, such as Colymboides minutus, don't appear to have been so specialized for diving. The humerus of Petralca, however, had very thick bone walls, which is characteristic of diving birds (including both extant loons and auks). It looks like Petralca truly lived up to its claim as a diver.

Tuesday, June 6, 2017

Near the base of Paraves where we once had only Archaeopteryx as a model for the last common ancestor of modern birds and dromaeosaurids, we now know of a plethora of protobirds that likely occupy the same general region of theropod phylogeny. The Late Jurassic Tiaojishan Formation in northeastern China has proven particularly productive in this regard, its fine-grained lake deposits playing host to Pedopenna, Anchiornis, Xiaotingia, Eosinopteryx, and Aurornis, with rumors of more to come.

Many of these early paravians are known from essentially complete remains, have their soft tissues preserved in exquisite detail, and, in the case of Anchiornis, are represented by hundreds of specimens, to the point where their integument, musculature, and even coloration can be restored with unprecedented accuracy. On the flip side, however, these specimens tend to be preserved as flattened corpses similar to roadkill, making aspects of their skeletal anatomy challenging to interpret.

Another geological formation exposed in northeastern China, the Early Cretaceous Yixian Formation, is also known for lake deposits containing "roadkill" fossil specimens with finely-preserved soft tissues. However, the Yixian has other fossil beds that were formed by volcanic ash, and though the fossils found in these generally lack obvious soft tissue structures, they can preserve complete skeletons in three dimensions. A newly-named basal paravian has been recovered from these Yixian ash beds, making it a potentially quite significant find.

The holotype of Liaoningvenator, from Shen et al. (2017).

Liaoningvenator curriei is known from a nearly complete specimen, and at a glance it has some striking features. It looks extremely leggy, with very long lower legs and and feet for its size. Based on rough measurements done in ImageJ, the longest metatarsal is around 64% the length of the tibia, comparable to the ratio seen in some cusorial noasaurids such as Elaphrosaurus. Its forelimbs, conversely, are quite short, the humerus being less than 60% the length of the femur. (Compare Jinfengopteryx, another short-armed basal paravian, in which this figure is around 70%.)

The tail also looks unusually short and the life restoration provided in the paper appears to take this at face value, depicting the preserved length of the tail as its total length in life. The tail as preserved looks truncated, at least to my eye, so I, for one, am skeptical of this interpretation. (The authors do not comment on this issue one way or another.) Even so, it may not be farfetched to suggest that even the complete tail of Liaoningvenator was fairly short, considering that the phylogenetic analysis in the description finds it to be the sister taxon of Eosinopteryx, which has a relatively short tail with only 20 tail vertebrae in total. (For comparison, Anchiornis and Aurornis both have around 30 tail vertebrae.)

Speaking of phylogenetic affinities, the description recovers Liaoningvenator and Eosinopteryx as part of a clade of basal troodontids along with Anchiornis and Xiaotingia. If these findings are valid, Liaoningvenator would be the geologically youngest known member of this basal clade. It would also be the largest member of the group by far: using the Christiansen and Fariña (2004) method of estimating theropod body mass by femur length, Liaoningvenator is predicted to have weighed around 2 kg, whereas its Tiaojishan brethren have all been estimated as less than 1 kg. (On top of that, bone histology indicates that the holotype of Liaoningvenator was still growing at the time of death, though approaching skeletal maturity.)

That's all very fascinating if true. However, the phylogenetic relationships of these basal paravians are notoriously difficult to figure out. Anchiornis (the best-studied Tiaojishan paravian) has bounced between being an avialan, a troodontid, and neither ever since its initial description. In addition, it is not clear whether all of these protobirds really do clade together to the exclusion of other paravians. Perhaps we should also expect a tumultuous phylogenetic future for Liaoningvenator.

I would be remiss if I neglected to mention that Liaoningvenator is not the only small basal paravian known from a complete, three-dimensionally preserved specimen. Mei is known from two suchspecimens, also found in ash beds from the Yixian Formation. Infamously, both of these specimens have been preserved in what appears to be a sleeping posture, which has led to jokes that Mei spent all of its time sleeping, despite efforts by paleoartists to depict alternative behaviors. Unlike Liaoningvenator, however, Mei has never been considered a close relative of the Tiaojishan "problem paravians", and is uniformly recovered as a troodontid.

Sunday, June 4, 2017

Being in the UK opens up access to a whole host of paleontological conferences that I might not have considered attending before. Among these is Progressive Palaeontology (ProgPal), a conference run specifically by grad students for grad students.

This year's ProgPal was held in the city of Leicester. The first afternoon of the conference was host to a couple of workshops, but the first event I attended was the icebreaker later in the evening, which took place at the New Walk Museum.

The paleontology exhibit at this museum is particularly rich in Jurassic marine fossils. Here are the magical jaws of Liopleurodon, poised to clamp down on the head and neck of Muraenosaurus.

As is also visible in the previous photo, many of the fossils were accompanied by small models representing life restorations.

Direct evidence that plesiosaurs ended up on the menu of other sea creatures, in the form of bite marks on some of their bones.

The skull of Rhomaleosaurus.

An ichthyosaur showing off a massive sclerotic ring.

The New Walk Museum is home to some newly-discovered marine reptiles, such as this currently unnamed plesiosaur.

Here is Wahlisaurus, an ichthyosaur that was only described last year.

Naturally, there are also a few dinosaurs on display, including this model of a Neovenator skeleton.

The centerpiece of the hall is the sauropod Cetiosaurus.

Some Ediacaran fossils get in on the act, including several specimens of Charnia.

This is said to be the only solo portrait that David Attenborough has agreed to sit.

The museum has some interesting osteological material of extant species as well, such as this skeleton of a tree kangaroo.

The skull of an Indian (or Ganges) softshell turtle.

The main events of the conference, however, happened on the second day. These were, naturally, the talks and poster presentations. I didn't present anything, given that I had barely begun to do my Master's research by the time abstracts were due. Next year, perhaps.

Even so, the Bristol contingent was well represented (unsurprisingly, considering the size of our research group), accounting for around a quarter of the total number of both attendees and presentations given. (Yes, I counted.) Dare I say, this may have been a double-edged sword in some ways; I noticed that many of us Bristolians (myself included) tended to cluster together during free periods rather than taking the time to meet new people. Nonetheless, I did have quite a few worthwhile interactions with some non-Bristolian delegates, such as meeting with future collaborator Juan Benito Moreno for the first time. I also received some (positive) attention for my continuous livetweeting of the conference talks. (Livetweeting at SVP has given me plenty of opportunities for practice.)

Most of the talks were livesteamed and can be viewed here (though the sound appears to be out of sync towards the end of the recording). Institutional loyalties aside, my favorite talks include (in order of presentation):

Virginia Harvey's talk on using ancient collagen to study the fauna of the Cayman Islands (which deservedly went on to win best talk)

Alessandro Chiarenza's talk on modelling the ecological niches of dinosaurs in the latest Cretaceous of North America

David Marshall's talk on the paleoecology of Acutiramus (a Bristol talk, but the fact that he had to give a hand-drawn presentation due to spending his weekend trying to salvage a vandalized fossil site and still knocked it out of the park means he more than deserves a mention)

On the whole, I had a great time at ProgPal and found it to be very effective at what it sets out to do (providing a relatively laid-back environment for early-career paleontologists to share their research). I intend to be back next year.